How to calculate buoyant force
A submerged object displaces liquid. The liquid pushes upward with a force equal to the weight of the displaced liquid, not automatically the weight of the object.
What the formula is saying
Find the mass of displaced liquid using ρV, then multiply by g to turn mass into weight. Use only the submerged displaced volume.
Read the symbols in plain language
- ρ
- Fluid densitykg/m³
- g
- Gravitym/s²
- V
- Displaced volumem³
Sort out the units first
Use the liquid density in kg/m³ and volume in m³. 20 litres = 0.02 m³. The resulting force is in N, not kg.
Let’s solve one together
Read the given values, follow each operation, then check what the result means.
- ρ · Fluid density
- 1000 kg/m³
- g · Gravity
- 9.81 m/s²
- V · Displaced volume
- 0.02 m³
Find the displaced liquid mass
Density belongs to the surrounding liquid, not the object.
(1000) × (0.02) = 20 kgConvert displaced mass to its weight
That weight equals the upward buoyant force.
(20) × (9.81) = 196.2 N
Does this worked answer make sense?
The example displaces 20 kg of water and therefore gives 196.2 N upward. Double the displaced volume: double the buoyancy.
Now try your own values
Change a value or its unit. The same method will show your calculation, step by step.
Results update only when you calculate. The lesson example above stays unchanged.
Your turn — check your understanding
Use these new values. Work it out first, then check your answer.
- ρ · Fluid density
- 1000 kg/m³
- g · Gravity
- 9.81 m/s²
- V · Displaced volume
- 0.05 m³
Find: buoyant force
A hint, not the answer
Find the mass of displaced liquid using ρV, then multiply by g to turn mass into weight. Use only the submerged displaced volume.
Use the liquid density in kg/m³ and volume in m³. 20 litres = 0.02 m³. The resulting force is in N, not kg.
Show the full practice solution
Compare the steps with your work; revealing a solution does not mark the lesson complete.
Find the displaced liquid mass
Density belongs to the surrounding liquid, not the object.
(1000) × (0.05) = 50 kgConvert displaced mass to its weight
That weight equals the upward buoyant force.
(50) × (9.81) = 490.5 N
Avoid the common trap
Using the object’s density gives the wrong force. A partially submerged object displaces less than its complete external volume.
When this method applies — and when it does not
Uniform-density fluid at rest. To decide whether the object accelerates, also compare its weight and any other forces.
For study and understanding, not approval of a real structure, site operation or design. Apply the correct standard, National Annex and professional review to actual engineering work.
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Sources & further reading
References open in a new tab and explain the underlying principles. The teaching text and examples here are SimpleFlick’s own; the source organisations have not endorsed this calculator.
Lesson updated: · Worked examples checked against the implemented formula; not an independent engineering certification.